Method for removing carbon dioxide from combustion exhaust gas

ABSTRACT

There are disclosed a method for removing CO 2  from a combustion exhaust gas which comprises the step of bringing the combustion exhaust gas under atmospheric pressure into contact with an aqueous solution of a hindered amine selected from the group consisting of 2-amino-2-methyl-1-propanol, 2-methylaminoethanol, 2-ethylamino-ethanol and 2-piperidineethanol; and another method for removing carbon dioxide from a combustion exhaust gas which comprises the step of bringing the combustion exhaust gas under atmospheric pressure into contact with a mixed aqueous solution of 100 parts by weight of an amine compound (X) selected from the group consisting of 2-amino-2-methyl-1, 3-propanediol, 2-amino-2-methyl-1-propanol, 2-amino-2-ethyl-1, 3-propanediol, t-butyldiethanolamine and 2-amino-2-hydroxymethyl-1,3-propanediol; and 1-25 parts by weight of an amine compound (Y) selected from the group consisting of piperazine, piperidine, morpholine, glycine, 2-methylamino-ethanol, 2-piperidineethanol and 2-ethylaminoethanol.

This is a continuation of application Ser. No. 08/867,988, filed Jun. 3,1997, now abandoned which, in turn, is a continuation of applicationSer. No. 08/741,582, filed Nov. 1, 1996, now abandoned, which, in turn,is a continuation of application Ser. No. 08/328,398, filed Oct. 24,1994, now abandoned, which, in turn, is a continuation of applicationSer. No. 08/021,378, filed Feb. 23, 1993, now abandoned.

FIELD OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a method for removing CO₂ (carbondioxide) from a combustion exhaust gas. More specifically, it relates toa method for removing CO₂ from a combustion exhaust gas underatmospheric pressure by the use of a specific mixed aqueous solutioncontaining an amine.

In recent years, a greenhouse effect by CO₂ is indicated as one cause ofthe warming phenomenon of the earth, and its prompt resolution isglobally required in order to protect earth circumstances. Thegeneration sources of CO₂ extend in active fields of all humans in whichfossil fuels are burned, and there is a tendency that the dischargeregulation of CO₂ will be further tightened in the future. Thus, forpower generation facilities such as thermoelectric power plants in whicha large amount of the fossil fuel is used, there are energeticallyresearched a method for removing and recovering CO₂ from a combustionexhaust gas by bringing the combustion exhaust gas coming from a boilerinto contact with an aqueous alkanolamine solution or the like, and amethod for storing the recovered CO₂ without discharging it into theatmosphere.

Examples of the alkanolamine include monoethanolamine, diethanolamine,triethanolamine, dimethyldiethanolamine, diisopropanolamine anddiglycolamine, and in general, monoethanolamine (abbreviated to “MEA”)is preferably used.

However, even if the above-mentioned aqueous alkanolamine solutiontypified by MEA is used as an absorbing solution for absorbing/removingCO₂ from a combustion exhaust gas, the effect of the alkanolamine is notalways satisfactory in view of an amount of absorbed CO₂ perpredetermined amount of the aqueous alkanolamine solution having apredetermined concentration, an amount of absorbed CO₂ per unit aminemole of the aqueous alkanolamine solution having a predeterminedconcentration, an absorption rate of CO₂ at a predeterminedconcentration, heat energy required to recover the aqueous alkanolaminesolution after the absorption, and the like.

In the meantime, for the separation of an acidic gas from various mixedgases by the use of an amine compound, many techniques are known.

Japanese Patent Application Laid-open No. 100180/1978 discloses a methodfor removing an acidic gas which comprises bringing a usually gaseousmixture into contact with an amine-solvent liquid absorbent comprising

(1) an amine mixture comprising at least 50 mole % of a steric hindranceamine constituting a part of a ring and having at least one secondaryamino group bonded to either of a secondary carbon atom or a tertiarycarbon atom or a primary amino group bonded to the tertiary carbon atom,and at least about 10 mole % of the tertiary amino-alcohol, and

(2) a solvent for the above-mentioned amine mixture which functions as aphysical absorbent for the acidic gas. Usable examples of the sterichindrance amine include 2-piperidine ethanol[2-(2-hydroxyethyl)-piperidine] and 3-amino-3-methyl-1-butanol, and ausable example of the tertiary amino-alcohol is3-dimethylamino-1-propanol. Furthermore, an example of the solvent is asulfoxide compound which may contain water in an amount of 25% by weightor less, and an example of a gas to be treated is “a usually gaseousmixture containing carbon dioxide and hydrogen sulfide at highconcentrations, for example, 35% Of CO₂ and 10-12% of H₂S” on page 1,left upper column of the same gazette. In the undermentioned examples,CO₂ itself is used.

In Japanese Patent Application Laid-open No. 71819/1986, there isdescribed a composition for the scraping of an acidic gas which containsa non-aqueous solvent such as a steric hindrance amine or sulfolane. Asan example of the primary monoamino alcohol of the steric hindrance,2-amino-2-methyl-1-propanol (abbreviated to AMP) is exemplified andused. In examples, CO₂ and nitrogen as well as CO₂ and helium are used.Furthermore, as absorbents, an aqueous solution of an amine andpotassium carbonate, and the like are used. The use of water is alsoreferred to. In addition, this gazette describes the advantage of thesteric hindrance amine in the absorption Of CO₂ by reaction formulae.

In Chemical Engineering Science, Vol. 41, No. 4, pp. 997-1,003, there isdisclosed a carbon dioxide gas absorption behavior of an aqueous AMPsolution which is a hindered amine. As gases to be absorbed, CO₂ and amixture of CO₂ and nitrogen at atmospheric pressure are used.

Chemical Engineering Science, Vol. 41, No. 4, pp. 405-408 has reportedabsorption rates of an aqueous solution of a hindered amine such as AMPand an aqueous solution of a straight-chain amine such as MEA to CO₂ andH₂S in the vicinity of ordinary temperature. According to this report, alarge difference is not present between both the aqueous solutions, inthe case that the partial pressure of CO₂ is 1 atm and theconcentrations of the aqueous solutions are from 0.1-0.3 mole. However,it is apparent that when the concentrations of the aqueous solutions are0.1 mole and the partial pressure of CO₂ is decreased to 1, 0.5 and 0.05atm, the absorption rate of AMP deteriorates more largely than that ofMEA at 0.05 atm.

U.S. Pat. No. 3,622,267 discloses a technique in which an aqueousmixture containing methyldiethanolamine and monoethylmonoethanolamine isused to purify a highpartial pressure CO₂ contained in a synthetic gassuch as a partially oxidized gas of a crude oil or the like, forexample, a synthetic gas containing 30% of CO₂ at 40 atm.

German Laid-open Patent No. 1,542,415 discloses a technique in which amonoalkylalkanolamine or the like is added to a physical or chemicalabsorbent in order to improve the absorption rate of CO₂, H₂S and COS.Similarly, German Laid-open Patent No. 1,904,428 discloses a techniquein which monomethylethanolamine is added for the purpose of improvingthe absorption rate of methyldiethanolamine.

U.S. Pat. No. 4,336,233 discloses a technique in which, for thepurification of a natural gas, a synthetic gas or a gasified coal gas, a0.81-1.3 mole/liter aqueous piperazine solution is used as a washliquid, or piperazine is used in the state of an aqueous solutiontogether with a solvent such as methyldiethanolamine, triethanolamine,diethanolamine or monomethylethanolamine as a wash liquid;

Similarly, Japanese Patent Application Laid-open No. 63171/1977discloses a CO₂ absorbent obtained by adding piperazine or a piperazinederivative such as hydroxyethylpiperazine as an accelerator to atertiary alkanolamine, a monoalkylalkanolamine or the like.

OBJECT AND SUMMARY OF THE INVENTION

As described above, a method for efficiently removing CO₂ from acombustion exhaust gas has been heretofore desired. In particular, inthe case that the combustion exhaust gas is treated with an aqueoussolution containing a CO₂ absorbent at a certain concentration, it is anurgent serious problem to select an absorbent which is capable ofabsorbing a large amount of CO₂ per unit mole of the absorbent andabsorbing a large amount of CO₂ per unit volume of the aqueous solutionand which has a high absorption rate. Furthermore, another requirementof the absorbent is to permit the separation of CO₂ and the recovery ofthe absorbing solution with a small amount of heat energy, after theabsorption of CO₂. Above all, it is desired to improve the absorptionrate of the absorbent having a large CO₂ absorption power but a lowabsorption rate.

In view of the above-mentioned problems, the present inventors haveintensively investigated an absorbent for use in the removal of CO₂ froma combustion exhaust gas. As a result, they have found that theemployment of a specific hindered amine is particularly effective, andthus the present invention has now been completed.

That is, the present invention is directed to a method for removingcarbon dioxide from a combustion exhaust gas which comprises the step ofbringing the combustion exhaust gas under atmospheric pressure intocontact with an aqueous solution of a hindered amine (exclusive of anamine having two or more amino groups) selected from the groupconsisting of:

(A) a compound having an alcoholic hydroxyl group and a primary aminogroup, said primary amino group being bonded to a tertiary carbon atomhaving two unsubstituted alkyl groups,

(B) a compound having an alcoholic hydroxyl group and a secondary aminogroup, said secondary amino group having an N atom bonded to a grouphaving a chain of 2 or more carbon atoms inclusive of a bonded carbonatom,

(C) a compound having an alcoholic hydroxyl group and a tertiary aminogroup, at least two groups bonded to said tertiary amino group having achain of 2 or more carbon atoms inclusive of a bonded carbon atom,respectively, two of the groups bonded to said tertiary amino groupbeing unsubstituted alkyl groups, and

(D) a 2-substituted piperidine having a hydroxyl group-substituted alkylgroup at the 2-position.

In the present invention, the above-mentioned hindered amine isparticularly preferably selected from the group consisting of2-amino-2-methyl-1-propanol, 2-(methyl-amino)-ethanol,2-(ethylamino)-ethanol, 2-(diethylamino)-ethanol and2-(2-hydroxyethyl)-piperidine.

Every hindered amine which can be used in the present invention has analcoholic hydroxyl group in its molecule. It is preferred that onealcoholic hydroxyl group is present in the hindered amine molecule.Further more, the molecular weight of the hindered amine is preferably150 or less from the viewpoint of CO₂ absorption power per unit amountof the solution at a predetermined concentration.

One of the hindered amines which can be used in the present invention is(A) a compound having an alcoholic hydroxyl group and a primary aminogroup, said primary amino group being bonded to a tertiary carbon atomhaving two unsubstituted alkyl groups. In this (A), the unsubstitutedalkyl groups may be mutually identical or different, and examples of theunsubstituted alkyl groups include a methyl group, an ethyl group and apropyl group, but both of the unsubstituted alkyl groups are preferablythe methyl groups. Typical examples of this (A) include2-amino-2-methyl-1-propanol, 3-amino-3-methyl-2-pentanol,2,3-dimethyl-3-amino-1-butanol, 2-amino-2-ethyl-1-butanol,2-amino-2-methyl-3-pentanol, 2-amino-2-methyl-1-butanol,3-amino-3-methyl-1-butanol, 3-amino-3-methyl-2-butanol,2-amino-2,3-dimethyl-3-butanol, 2-amino-2,3-dimethyl-1-butanol and2-amino-2-methyl-1-pentanol, and above all, 2-amino-2-methyl-1-propanol(AMP) is preferable.

Another amine of the hindered amines which can be used in the presentinvention is (B) a compound having an alcoholic hydroxyl group and asecondary amino group, said secondary amino group having an N atombonded to a group having a chain of 2 or more carbon atoms inclusive ofa bonded carbon atom. In this (B), the group having the chain of 2 ormore carbon atoms inclusive of the bonded carbon atom is usually ahydroxy group-substituted alkyl group of 2-5 carbon atoms, preferably analkyl group of 2-3 carbon atoms which may be substituted by the hydroxylgroup. Typical examples of this (B) include 2-(ethylamino)-ethanol,2-(methylamino)-ethanol, 2-(propylamino)-ethanol,2-(isopropylamino)-ethanol, 1-(ethylamino)-ethanol,1-(methylamino)-ethanol, 1-(propylamino)-ethanol and1-(isopropylamino)-ethanol, and above all, 2-(ethylamino)-ethanol(hereinafter abbreviated to “EAE”) and 2-(methyl-amino)-ethanol(hereinafter abbreviated to “MAE”) are preferably used.

Still another amine of the hindered amines which can be used in thepresent invention is (C) a compound having an alcoholic hydroxyl groupand a tertiary amino group, at least two groups bonded to said tertiaryamino group having a chain of 2 or more carbon atoms inclusive of abonded carbon atom, respectively, two of the groups bonded to saidtertiary amino group being unsubstituted alkyl groups. In this (C), thetwo unsubstituted alkyl group may be mutually identical or different,and examples of the unsubstituted alkyl groups include a methyl group,an ethyl group, a propyl group and an isopropyl group. Typical examplesof this (C) include 2-(dimethylamino)-ethanol, 2-(diethylamino)-ethanol,2-(ethylmethylamino)-ethanol, 1-(dimethylamino)-ethanol,1-(diethylamino)-ethanol, 1-(ethylmethylamino)-ethanol,2-(diisopropylamino)-ethanol, 1-(diethylamino)-2-propanol and3-(diethylamino)-1-propanol, and above all, 2-(diethylamino)-ethanol(herein after abbreviated to “DEAE”) is preferably used.

Still another amine of the hindered amines which can be used in thepresent invention is (D) a 2-substituted piperidine having a hydroxylgroup-substituted alkyl group at the 2-position. Typical examples of the2-substituted piperidine include 2-(hydroxymethyl)-piperidine,2-(2-hydroxyethyl)-piperidine and 2-(1-hydoxymethyl)-piperidine, andabove all, 2-(2-hydroxyethyl)-piperidine (hereinafter abbreviated to“HEP”) is preferable.

The hindered amines for use in the present invention selected from theabove-mentioned groups can be utilized singly or in the form of amixture.

The concentration of the aqueous hindered amine solution which can beused as an absorbing solution is usually from 25 to 65% by weight,depending upon the kind of hindered amine. The temperature of theaqueous hindered amine solution at the time of the contact with thecombustion exhaust gas is usually in the range of 30 to 70° C.

If necessary, a corrosion inhibitor, a hindered amine aging inhibitorand the like can be added to the aqueous hindered amine solution. Asthese inhibitors, conventionally usable inhibitors can be used.

In this connection, the expression “under atmospheric pressure” in thepresent invention covers a pressure range including the vicinity of theatmospheric pressure which permits the function of a blower or the likefor feeding the combustion exhaust gas.

In view of the above-mentioned problems, the present inventors haveintensively investigated an absorbent for use in the removal of CO₂ fromthe combustion exhaust gas, and as a result, they have found that theutilization of a mixture obtained by mixing a specific amine compound(X) with a relatively small amount of a specific amine compound (Y) isparticularly effective to improve the absorption rate of the specificamine compound (X). In consequence, the present invention has now beenachieved.

That is, the present invention is directed to a method for removingcarbon dioxide from a combustion exhaust gas which comprises the step ofbringing the combustion exhaust gas under atmospheric pressure intocontact with a mixed aqueous solution of 100 parts by weight of an aminecompound (X) selected from the group consisting of (A) a compound havingone alcoholic hydroxyl group and a primary amino group in its molecule,said primary amino group being bonded to a tertiary carbon atom havingtwo unsubstituted alkyl groups, (B) a compound having one alcoholichydroxyl group and a tertiary amino group in its molecule, at least twogroups bonded to said tertiary amino group having a chain of 2 or morecarbon atoms inclusive of a bonded carbon atom, respectively, two of thegroups bonded to said tertiary amino group being unsubstituted alkylgroups, and (C) diethanolamine; and 1-25 parts by weight of an aminecompound (Y) selected from the group consisting of (D) piperazine, (E)piperidine, (F) morpholine, (G) glycine, (H) 2-piperidinoethanol, and(I) a compound having one alcoholic hydroxyl group and a secondary aminogroup in its molecule, said secondary amino group having anunsubstituted alkyl group of 3 or less carbon atoms and an N atom bondedto a group having a chain of 2 or more carbon atoms inclusive of abonded carbon atom.

As a particularly preferable embodiment of the present invention, therecan be recited a method for removing CO₂ from a combustion exhaust gaswhich comprises the step of bringing the combustion exhaust gas underatmospheric pressure into contact with a mixed aqueous solution of 100parts by weight of an amine compound, as the above-mentioned aminecompound (X), selected from the group consisting of 2-amino-2-methyl-1,3-propanediol, 2-amino-2-methyl-1-propanol, 2-amino-2-ethyl-1,3-propanediol, t-butyldiethanolamine and2-amino-2-hydroxymethyl-1,3-propanediol; and 1-25 parts by weight of anamine compound, as the above-mentioned amine compound, selected from thegroup consisting of piperazine, piperidine, morpholine, glycine,2-methylaminoethanol, 2-piperidineethanol and 2-ethylaminoethanol.

The combination of the specific amine compounds (X) and (Y) which can beused in the present invention are as described above. However, one aminecompound (X) may be combined with one amine compound (Y), oralternatively, one of either group of the amine compounds (X) and (Y)may be combined with two or more of the other group.

One of the amine compounds which can be used in the present invention is(A) a compound having one alcoholic hydroxyl group and a primary aminogroup in its molecule, said primary amino group being bonded to atertiary carbon atom having two unsubstituted alkyl groups. In this (A),the unsubstituted alkyl groups may be mutually identical or different,and their examples include a methyl group, an ethyl group and a propylgroup. Preferably, both of the unsubstituted alkyl groups are the methylgroups. Typical examples of this (A) include2-amino-2-methyl-1-propanol, 3-amino-3-methyl-2-pentanol,2,3-dimethyl-3-amino-1-butanol, 2-amino-2-ethyl-1-butanol,2-amino-2-methyl-3-pentanol, 2-amino-2-methyl-1-butanol,3-amino-3-methyl-1-butanol, 3-amino-3-methyl-2-butanol,2-amino-2,3-dimethyl-3-butanol, 2-amino-2,3-dimethyl-1-butanol and2-amino-2-methyl-1-pentanol. Above all, 2-amino-2-methyl-1-propanol(AMP) is preferable.

Another compound of the amine compounds which can be used in the presentinvention is (B) a compound having one alcoholic hydroxyl group and atertiary amino group in its molecule, at least two groups bonded to thetertiary amino group having a chain of 2 or more carbon atoms inclusiveof a bonded carbon atom, respectively, two of the groups bonded to thetertiary amino group being unsubstituted alkyl groups. In this (B), thetwo unsubstituted alkyl group may be mutually identical or different,and their examples include a methyl group, an ethyl group, a propylgroup and an isopropyl group. Typical examples of this (B) include2-(dimethylamino)-ethanol, 2-(diethyl-amino)-ethanol,2-(ethylmethylamino)-ethanol, 1-(dimethyl-amino)-ethanol,1-(diethylamino)-ethanol, 1-(ethylmethylamino)-ethanol,2-(diisopropylamino)-ethanol, 1-(diethylamino)-2-propanol and3-(diethylamino)-1-propanol, and above all, 2-(diethylamino)-ethanol(hereinafter abbreviated to “DEAE”) are preferable.

Still another compound of the amine compounds which can be used in thepresent invention is (I) a compound having one alcoholic hydroxyl groupand a secondary amino group in its molecule, the secondary amino grouphaving an unsubstituted alkyl group of 3 or less carbon atoms and an Natom bonded to a group having a chain of 2 or more carbon atomsinclusive of a bonded carbon atom. In this (I), an example of the chainof 2 or more carbon atoms inclusive of the bonded carbon atom is usuallyan hydroxyl group-substituted alkyl group of 2-5 carbon atoms,preferably an hydroxyl group-substituted alkyl group of 2-3 carbonatoms. Typical examples of this (I) include 2-(ethylamino)-ethanol,2-(methylamino)-ethanol, 2-(propylamino)-ethanol,2-(isopropylamino)-ethanol, 1-(ethylamino)-ethanol,1-(methylamino)-ethanol, 1-(propylamino)-ethanol and1-(isopropylamino)-ethanol, and above all, 2-(ethylamino)-ethanol and2-(methylamino)-ethanol (hereinafter abbreviated to “MAE”) arepreferably used.

With regard to a mixing ratio of the amine compounds (X) and (Y), theamine compound (Y) is in the range of 1 to 25 parts by weight,preferably in the range of 1 to 10 parts by weight, based on 100 partsby weight of (X) in the case that the amine compound (X) comprises (A)and/or (B). Furthermore, the amine compound (Y) is in the range of 1 to25% by weight, preferably in the range of 10 to 25% by weight, based on100 parts by weight of (X) in the case that the amine compound (X)comprises (C) diethanol. The concentration of the amine compound (X) inthe mixed aqueous solution (which may be called an absorbing solution)is usually from 15 to 65% by weight, depending upon the kind of (X). Thetemperature of the mixed aqueous solution at the time of the contactwith the combustion exhaust gas is usually in the range of 30 to 70° C.

If necessary, a corrosion inhibitor, a hindered amine aging inhibitorand the like can be added to the mixed aqueous solution which can beused in the present invention.

In this connection, the expression “under atmospheric pressure” in thepresent invention covers a pressure range including the vicinity of theatmospheric pressure which permits the function of a blower or the likefor feeding the combustion exhaust gas.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet of one example of processes which can be employedin the present invention.

FIG. 2 is a graph showing a relation between the absorption of anabsorbing solution (Nm³ of CO₂/m³ of absorbing solution, ordinate axis)and a temperature (° C., abscissa axis) in Examples 1 to 4 andComparative Example 1.

FIG. 3 shows an addition effect of an amine compound (Y) in an absorbingsolution using DEA in Examples 2 to 8.

FIG. 4 is a graph showing a relation between the absorption of anabsorbing solution (Nm³ of CO₂/m³ of absorbing solution, ordinate axis)and a temperature (° C., abscissa axis) in Example 6.

FIG. 5 shows an addition effect of piperazine in an absorbing solutionusing DEAE in Example 9.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

No particular restriction is put on a process which can be employed in amethod for removing CO₂ from a combustion exhaust gas according to thepresent invention, and its one example will be described in reference toFIG. 1. In FIG. 1, only main facilities are shown, and attachmentdevices are omitted.

In FIG. 1, numeral 1 is a CO₂ removing tower, 2 is a lower fillingportion, 3 is an upper filling portion or a tray, 4 is a CO₂ removingtower combustion exhaust gas feed opening, 5 is a CO₂-free combustionexhaust gas discharge opening, 6 is an absorbing solution feed opening,7 is a nozzle, 8 is a combustion exhaust gas cooler which can beprovided when needed, 9 is a nozzle, 10 is a filling portion, 11 is ahumidifying/cooling water circulating pump, 12 is a supplemental waterfeed line, 13 is a CO₂-containing absorbing solution discharge pump, 14is a heat exchanger, 15 is an absorbing solution reproducing tower(which can also simply be called “reproducing tower”), 16 is a nozzle,17 is a lower filling portion, 18 is a reproducing heater (reboiler), 19is an upper filling portion, 20 is a reflux water pump, 21 is a CO₂separator, 22 is a recovered CO₂ discharge line, 23 is a reproducingtower reflux condenser, 24 is a nozzle, 25 is a reproducing tower refluxwater feed line, 26 is a combustion exhaust gas feed blower, 27 is acooler, and 28 is a reproducing tower reflux water feed opening.

In FIG. 1, the combustion exhaust gas is introduced into the combustionexhaust gas cooler 8 by means of the combustion exhaust gas feed blower26, brought into contact with humidifying/cooling water from the nozzle9 in the filling portion 10, humidified/cooled therein, and then led tothe CO₂ removing tower 1 through the CO₂ removing tower combustionexhaust gas feed opening 4. The humidifying/cooling water which has beenbrought into contact with the combustion exhaust gas is stored in thelower portion of the combustion exhaust gas cooler 8, and it is thencirculated to the nozzle 9 by means of the pump 11 and used again. Thehumidifying/cooling water is gradually lost, while used to humidify/coolthe combustion exhaust gas, and therefore it is replenished through thesupplemental water feed line 12. When the humidified/cooled combustionexhaust gas is further cooled in view of the state of this gas, a heatexchanger can be disposed between the humidifying/cooling watercirculating pump 11 and the nozzle 9 to cool the humidifying/coolingwater, and the thus cooled water can be then fed to the combustionexhaust gas cooler

The combustion exhaust gas introduced into the CO₂ removing tower 1 isbrought into counterflow contact with the absorbing solution having apredetermined concentration fed from the nozzle 7 in the lower fillingportion 2, whereby CO₂ in the combustion exhaust gas is absorbed/removedby the absorbing solution. Afterward, the CO₂-free combustion exhaustgas streams toward the upper filling portion 3. The absorbing solutionfed to the CO₂ removing tower 1 absorbs CO₂, and the temperature of theabsorbing solution becomes higher than a temperature thereof at the feedopening 6 owing to absorption heat generated by the absorption. Theabsorbing solution is then forwarded to the heat exchanger 14 by meansof the CO₂-containing absorbing solution discharge pump 13, and it isheated and then led to the absorbing solution reproducing tower 15. Thetemperature adjustment of the reproduced absorbing solution can becarried out by the heat exchanger 14 or the cooler 27 disposed betweenthe heat exchanger 14 and the feed opening 6, if necessary.

In the absorbing solution reproducing tower 15, the absorbing solutionis reproduced in the lower filling portion 17 by heating of thereproducing heater 18, and then cooled by the heat exchanger 14, andthen returned to the CO₂ removing tower 1. In the upper portion of theabsorbing solution reproducing tower 15, CO₂ separated from theabsorbing solution is brought into contact with reflux water fed fromthe nozzle 24 in the upper filling portion 19, and then cooled by thereproducing tower reflux condenser 23. Afterward, in CO₂ separator 21,CO₂ is separated from reflux water formed by the condensation of watervapor accompanied with CO₂, and then led to a CO₂ recovery processthrough the recovered CO₂ discharge line 22. A large part of the refluxwater is refluxed to the absorbing solution reproducing tower 15 bymeans of the reflux water pump 20, and a small part of the reflux wateris fed to the reproducing tower reflux water feed opening 28 of the CO₂removing tower 1 through the reproducing tower reflux water feed line25. Since this reproducing tower reflux water contains a small amount ofthe absorbing solution, it is brought into contact with the exhaust gasin the upper filling portion 3 of the CO₂ removing tower 1, therebycontributing to the removal of a small amount of CO₂ contained in theexhaust gas.

Now, the present invention will be described in detail in reference toexamples.

EXAMPLES 1 to 5, COMPARATIVE EXAMPLE 1

50 ml of an absorbing solution comprising a 30% by weight aqueoushindered amine solution was placed in a glass reaction vessel (flask)disposed in a thermostatic chamber, and a mixed gas (a test gas) was fedto the flask with stirring at 40° C. at a flow rate of 1 liter/minuteunder atmospheric pressure. The test gas used herein was a modelcombustion exhaust gas (which corresponds to an LNG-fired exhaust gas)at 40° C. having a composition of 10 mole % of CO₂, 3 mole % of O₂ and87 mole % of N₂.

The test gas was continuously fed thereto, and when the CO₂concentration of the fed gas was equal to that of the discharged gas,CO₂ contained in the absorbing solution was measured by the use of a CO₂analyzer (a total organic carbon meter) to measure a CO₂ absorption in asaturation state. Similar tests were carried out at temperatures of 60°C. and 80° C.

Furthermore, for comparison, a 30% by weight aqueous MEA solution wassimilarly used.

The obtained results are set forth in Table 1 (the results at 40° C.)and in FIG. 2. In FIG. 2, the unit of an ordinate axis is Nm³ of CO₂/m³of the aqueous solution, and that of an abscissa axis was a temperature(° C.).

A tangential gradient at the beginning of the gas feed was calculatedfrom a relation graph between the CO₂ concentration of the gas at aflask outlet and a gas feed time, and a CO₂ initial absorption rate ofthe absorbing solution was obtained in a ratio to that of the aqueousMEA solution at the same concentration.

TABLE 1 Test Absorption of CO₂ Absorbing in Saturation State InitialSolution Nm³ of Absorp- (30 wt % mole of CO₂/m³ of tion Rate AqueousCO₂/mole Absorbing (MEA Aqueous Solution) of Amine Solution Solution= 1) Comp. Ex. 1 MEA 0.56 61.2 1.00 Example 1 AMP 0.72 54.4 0.69 Example2 MAE 0.63 56.0 1.00 Example 3 EAE 0.68 51.3 0.91 Example 4 DEAE 0.7542.2 0.36 Example 5 HEP 0.84 43.9 0.90

As is apparent from the results in Table, 1, the initial absorbing ratesof the aqueous hindered amine solutions which are the absorbingsolutions of the present invention are not so low as to be anticipatedexcept DEAE, and they are at such levels as to be equal to or a littlesmaller than MEA. It is possible to improve the absorption rate by theaddition of an absorption accelerator.

On the other hand, the absorption of CO₂ per unit mole of the hinderedamine is larger in all the cases of the aqueous hindered amine solutionsthan in the case of MEA. In addition, the absorption of CO₂ per unitvolume of the absorbing solution is slightly smaller in the cases of MAEand AMP than in the case of MEA, depending upon the kind of hinderedamine.

It is apparent from FIG. 2 that the absorption of CO₂ decreases morelargely with the elevation of an absorbing solution temperature in thecases the hindered amines typified by AMP than in the case of MEA. Thisindicates that heat energy can be more saved in the reproduction of theabsorbing solution in the cases of these hindered amines than in thecase of MEA.

As described above, when a combustion exhaust gas under atmosphericpressure is treated with a specific aqueous hindered amine solution asan absorbing solution in accordance with the present invention, theremoval of CO₂ can efficiently be achieved from the viewpoints ofabsorption power and the reproduction energy of the absorbing solution,though the CO₂ absorption rate of the aqueous hindered amine solutionregarding the present invention is equal to or slightly lower than thatof MEA.

EXAMPLES 6 to 8, COMPARATIVE EXAMPLES 2 to 4

50 ml of an absorbing solution, i.e., an aqueous solution prepared bymixing an amine compound (X) selected from 2-amino-2-methyl-1-propanol(AMP), diethanolamine (DEA) and monoethanolamine (MEA) with an aminecompound (Y) selected from 2-(methylamino)-ethanol (MAE) and piperazinein each ratio shown in Table 2 was placed in a glass reaction vessel(flask) disposed in a thermostatic chamber, and a mixed gas (a test gas)was fed to the flask with stirring at 40° C. at a flow rate of 1liter/minute under atmospheric pressure. The test gas used herein was amodel combustion exhaust gas (which corresponds to an LNG-fired exhaustgas) at 40° C. having a composition of 10 mole % of CO₂, 3 mole % of O₂and 87 mole % of N₂.

The test gas was continuously allowed to stream, and when the CO₂concentration of the fed gas was equal to that of the discharged gas,CO₂ contained in the absorbing solution was measured by the use of a CO₂analyzer (a total organic carbon meter) to measure the absorption of CO₂in a saturation state (Nm³ of CO₂/m³ of the absorbing solution, and moleof CO₂/mole of the absorbing solution).

A tangential gradient at the beginning of the gas feed was calculatedfrom a relation graph between the CO₂ concentration of the gas at aflask outlet and a gas feed time, and a CO₂ initial absorption rate ofthe absorbing solution was obtained in a ratio to an initial absorptionrate in the aqueous MEA solution at the same concentration as in theamine compound (X).

The same test as described above was carried out at 60° C.,

Furthermore, for comparison, each single solution of the amine compounds(X), i.e., MEA, DEA and AMP was subjected to an absorption test at 40°C., 60° C. and 80° C.

The obtained results are set forth in Tables 2 to 4 and FIG. 3. ThisFIG. 3 shows an addition effect of the amine compound (Y) at atemperature of 40° C. in the case that DEA was used as the aminecompound (X), and in this drawing, an abscissa axis was theconcentration of MAE and piperazine as the amine compounds (Y) and anordinate axis was an absorption reaction rate ratio.

TABLE 2 CO₂ Absorption of Absorbing Test Solution in Saturation Stat(Nm³ of CO₂/m³ of Solution) Absorption of CO₂ in Saturation State (Nm³of Absorbing CO₂/m³ of Solution) Test Solution (wt %) 40° C. 60° C.Comp. Monoethanolamine 30% 61.15 56.45 Example 2 (MEA) 45% 89.15 79.7460% 101.70 105.73 Comp. Diethanolamine 30% 34.27 23.30 Example 3 (DEA)45% 47.04 34.94 60% 56.45 43.90 Comp. 2-amino-2-methyl- 30% 54.43 33.38Example 4 1-propanol (AMP) Example 6 AMP + (1) 30% 56.22 37.41 AMP + (2)30% 62.50 48.16 Example 7 DEA + (1) 30% 36.74 28.45 45% 49.50 37.86 60%62.94 48.16 Example 8 DEA + (2) 30% 46.59 37.18 45% 59.81 46.37 60%72.13 57.34 AMP + (1): 50 ml of AMP test solution + 1.5 g (0.02 mole) ofMAE AMP + (2): 50 ml of AMP test solution + 3.34 g (0.02 mole) ofpiperazine DEA + (1): 50 ml of DEA test solution + 1.5 g (0.02 mole) ofMAE DEA + (2): 50 ml of DEA test solution + 3.34 g (0.02 mole) ofpiperazine

TABLE 3 CO₂ Absorption of Absorbing Test Solution in Saturation Stat(mole of CO₂/mole of Solution) Absorption of CO₂ in Saturation State(mole of Absorbing CO₂/mole of Soln.) Test Solution (wt %) 40° C. 60° C.Comp. Monoethanolamine 30% 0.56 0.51 Example 2 (MEA) 45% 0.54 0.48 60%0.46 0.48 Comp. Diethanolamine 30% 0.54 0.36 Example 3 (DEA) 45% 0.490.36 60% 0.44 0.34 Comp. 2-amino-2-methyl- 30% 0.72 0.44 Example 41-propanol (AMP) Example 6 AMP + (1) 30% 0.67 0.44 AMP + (2) 30% 0.670.52 Example 7 DEA + (1) 30% 0.50 0.39 45% 0.47 0.36 60% 0.46 0.35Example 8 DEA + (2) 30% 0.57 0.54 45% 0.53 0.41 60% 0.49 0.39 AMP + (1):50 ml of AMP test solution + 1.5 g (0.02 mole) of MAE AMP + (2): 50 mlof AMP test solution + 3.34 g (0.02 mole) of piperazine DEA + (1): 50 mlof DEA test solution + 1.5 g (0.02 mole) of MAE DEA + (2): 50 ml of DEAtest solution + 3.34 g (0.02 mole) of piperazine

TABLE 4 Absorption Reaction Initial Rate Ratio of Absorbing TestSolution Ratio of Absorption Absorbing Reaction Initial Rate TestSolution (wt %) 40° C. 60° C. Comp. Monoethanolamine 30% 1.00 1.03Example 2 (MEA) 45% 1.00 1.06 60% 0.97 1.05 Comp. Diethanolamine 30%0.64 0.84 Example 3 (DEA) 45% 0.69 0.87 60% 0.47 0.82 Comp.2-amino-2-methyl- 30% 0.69 0.95 Example 4 1-propanol (AMP) Example 6AMP + (1) 30% 0.95 1.00 AMP + (2) 30% 0.97 0.97 Example 7 DEA + (1) 30%0.86 0.95 45% 0.77 0.91 60% 0.63 0.82 Example 8 DEA + (2) 30% 0.94 1.0045% 0.86 1.01 60% 0.81 0.94 AMP + (1): 50 ml of AMP test solution + 1.5g (0.02 mole) of MAE AMP + (2): 50 ml of AMP test solution + 3.34 g(0.02 mole) of piperazine DEA + (1): 50 ml of DEA test solution + 1.5 g(0.02 mole) of MAE DEA + (2): 50 ml of DEA test solution + 3.34 g (0.02mole) of piperazine

Furthermore, FIG. 4 shows a relation between the absorption of CO₂ (Nm³of CO₂/m³ of the absorbing solution) and temperature. It is apparentfrom FIG. 4 that the absorption of CO₂ decreases more largely with theelevation of the absorbing solution temperature in the case that amixture of AMP and 2-methylaminoethanol was used than in the case thatMEA was used. This indicates that heat energy can be more saved in thereproduction of the absorbing solution in the case of the mixture of AMPand 2-methylaminoethanol than in the case of MEA.

EXAMPLE 9, COMPARATIVE EXAMPLE 5

In order to inspect an effect of a reaction accelerator, a test was madeby the use of a wet wall type absorbing tower having a diameter of 15 mmand a length of 7.5 mm which was the model of an absorbing tower used inan actual process. In this test, a 30% by weight DEAE solution was usedas an absorbing solution, an actual boiler combustion exhaust gas (CO₂=9mole %, O₂=2 mole %, water vapor=saturation state, N₂=mole % of balance)was used as a combustion exhaust gas, an L/G was 2.0 liters/m³N, aliquid temperature and a gas temperature were both maintained at 40° C.,and a gas flow rate was 3.1 m/sec. The results are set forth in FIG. 5.In this FIG. 5, an ordinate axis was an absorption ratio of CO₂ absorbedfrom the fed combustion exhaust gas, and an abscissa axis was aconcentration of piperazine added to the absorbing solution. Theabsorption reaction acceleration effect of piperazine to DEAE wasapparent from FIG. 5.

As is definite from the above-mentioned results, when an amine compound(X) is mixed with a relatively small amount of an amine compound (Y) andthen used in accordance with the present invention, an initialabsorption rate can be improved more largely than in the case that theamine compound (X) is singly used. In addition, the absorption of CO₂per unit mole of (X) of a mixed absorbing solution is larger than in thecase that MEA is used.

As described above, when a mixed aqueous solution of a specific aminecompound (X) and a specific amine compound (Y) is used as an absorbingsolution for a combustion exhaust gas under atmospheric pressure inaccordance with a method of the present invention, an absorption rate ofCO₂ can be improved more largely than when the amine compound (X) issingly used. Additionally, according to the present invention, theremoval of CO₂ can be more efficiently achieved from the viewpoints ofabsorption and reproduction energy than when MEA is used.

What is claimed is:
 1. A method for removing carbon dioxide from acombustion exhaust gas containing oxygen gas which comprises the step ofbringing said combustion exhaust gas under atmospheric pressure intocontact with an aqueous solution consisting essentially of an absorbentselected from the group consisting of 2-(methylamino)-ethanol and2-ethylamino ethanol.
 2. A method for removing carbon dioxide from acombustion exhaust gas containing oxygen gas consisting of the step ofbringing the combustion exhaust gas under atmospheric pressure intocontact with a mixed aqueous solution of 100 parts by weight of2-amino-2-methyl-1-propanol; and 1-25 parts by weight of piperazine. 3.A method of removing carbon dioxide from a combustion exhaust gascontaining oxygen gas which comprises the step of bringing thecombustion exhaust gas under atmospheric pressure into contact with amixed aqueous solution of 100 parts by weight of an amine compoundselected from the group consisting of 2-amino-2-methyl-1, 3-propanediol,2-amino-2-ethyl-1, 3-propanediol and2-amino-2-hydroxymethyl-1,3-propanediol; and 1-25 parts by weight of an amine compound selectedfrom the group consisting of morpholine, glycine,2-(methylamino)-ethanol and 2-(2-hydroxyethyl)piperdine.
 4. The methodof claim 1, wherein said combustion exhaust gas contains from about 9 toabout 10% by volume of CO₂ and about 2 to about 3% by volume of O₂. 5.The method of claim 2, wherein said combustion exhaust gas contains fromabout 9 to about 10% by volume of CO₂ and about 2 to about 3% by volumeof O₂.
 6. The method of claim 3, wherein said combustion exhaust gascontains from about 9 to about 10% by volume of CO₂ and about 2 to about3% by volume of O₂.
 7. The method of claim 1, wherein the concentrationof the absorbent is 25-65% by weight.